1. Field of the Invention
The invention relates to a controller of a DC-DC converter, and more particularly to a DC-DC converter with a constant on time (COT) pulse width modulation (PWM) controller.
2. Description of the Related Art
DC-DC converters are widely used for various electronic devices. A constant on-time (COT) voltage regulator is one type of DC-DC converter. In general, a COT voltage regulator may turn on a main switch during a fixed period when a feedback voltage is smaller than a reference voltage, and the COT voltage regulator may adjust a turn off period of the main switch so that a steady output voltage may be provided. An output capacitor with a high equivalent series resistance (ESR) disposed in parallel with a load is necessary for a conventional COT voltage regulator, so that a steady output voltage may be provided. However, although a high ESR may help to provide system stability, for a COT voltage regulator, probability of output ripples increase due to the high ESR, which negatively influence the output voltage and power conversion efficiency of the COT voltage regulator.
U.S. Pat. No. 6,583,610 discloses a voltage regulator which operates in ripple-mode and comprises a virtual ripple generator. The virtual ripple generator provides a regulator feedback signal that includes a generated ripple component as a composite signal. The composite signal is generated according to an actual output signal and a ripple signal synchronized to switching cycles of the voltage regulator. Thus, the regulation feedback signal reflects the DC value of the output signal and is responsive to transient changes in the output signal level.